One way to approach an essay on this novel is to analyze the elements that reflect the Naturalism of Zola. Naturalism followed Realism and was initiated with Zola's efforts to add scientific precision to realistic descriptions. He also did much to incorporate not just common people as characters, but socially "low" people as characters. Your thesis if such an approach might be followed would have to express a debatable opinion about the success of his method; the moral value or literary value of his method; the direction literature took as a result of his method; or some other opinion or assertion that will result in controversy that merits discussion or further research.